We do a combo of things from above. Courtesy lights at the front and in the back are turned and we tend not to switch on the ignition / stereo when camped (no need for them as we have a rechargeable stereo bluetooth speaker which works with phones/ipads for music and internet radio as well as acting as a 'soundbar' for decent TV audio. I also carry a small and cheap (IIRC about £16) Lidl 'smart' battery charger in the van and keep an eye on the voltage of the starter battery - if the charge starts to go down after a few days, I plug it into leisure 240V and 12V dash socket for a couple of hours and it's all sorted again. The other option, as recommended by
@Drive Wayne s to fit an Ablemail AMT12-2 which will put a trickle charge into the starter battery from the leisure system - I've been contemplating one of those for ages but need a lot of reassurance that wiring it in is an easy DIY job for a simpleton like me.